In the case of using a compressor to supply cathode gas to a fuel cell stack, the compressor has to supply more cathode gas than the amount of air to be supplied to the fuel cell stack to avoid the surge of the compressor and the like in some cases. However, the fuel cell stack does not require the amount of air other than that necessary for power generation and wetness control. Thus, JP2009-123550A discloses a conventional fuel cell system in which, out of cathode gas discharged from a compressor, the amount of air unnecessary for a fuel cell stack is discharged to a cathode gas discharge passage via a bypass passage.